Electrophotographic dry developing systems employed in a copying machine, MFP or other types of electrophotographic apparatuses are categorized into a system using a one-component developer containing only toner and a system using a two-component developer containing toner and carrier. In either of these developing systems, toner charged to a predetermined level is applied to a photoreceptor. An electrostatic latent image formed on the photoreceptor is rendered visual with the toner and is transferred to a sheet of paper. The image visualized by the toner is fixed on the paper to obtain a desired image.
A brief description about development with the two-component developer will be given. A predetermined amount of toner and a predetermined amount of carrier are accommodated in a developing apparatus. The developing apparatus is provided with a plurality of rotatable magnet rollers, which are arranged circumferentially to present alternative south and north poles, and an agitation roller for agitating and mixing the toner and carrier in the developing apparatus. The carrier made of a magnetic powder is carried by the magnet rollers. The magnetic force of the magnet rollers forms a straight-chain like magnetic brush of carrier particles. Agitation produces triboelectric charges that bond a plurality of toner particles to the surface of the carrier particles. The magnetic brush abuts against the photoreceptor with rotation of the magnet rollers and supplies the toner to the surface of the photoreceptor. Development with the two-component developer is carried out as described above.
Fixation of the toner on a sheet of paper results in successive consumption of toner in the developing apparatus, and new toner in the same amount as that of the consumed toner is supplied, whenever needed, from a toner hopper attached to the developing apparatus. On the other hand, the carrier is not consumed for development and used as it is until the carrier comes to the end of its life. The carrier, which is a component of the two-component developer, is required to have various functions including: a function of triboelectrically charging the toner by agitation in an effective manner; an insulating function; and a toner transferring ability to appropriately transfer the toner to the photoreceptor. To improve the toner charging performance, for example, the carrier is required to have appropriate electric resistance (hereinafter, sometimes simply referred to as “resistance”) and appropriate insulating properties.
The aforementioned carrier currently made is composed of a carrier core particle, which is a core or a base of the carrier, and a coating resin, which covers the surface of the carrier core particle.
The carrier core particle is desired to have high mechanical strength as basic characteristics. As described above, the carrier is agitated in the developing apparatus, and it is desirable to prevent the carrier from being chipped and cracked by agitation as much as possible. Accordingly, the carrier core particle covered with coating resin is also desired to have high mechanical strength.
In addition, the carrier core particle is desired to have good magnetic properties. Briefly speaking, the carrier is carried by magnet rollers with magnetic force in the developing apparatus. Under the usage, if the magnetism, more specifically, the magnetization of the carrier core particle is low, the retention of the carrier to the magnet rollers becomes low, which may cause so-called scattering of the carrier or other problems. Especially, recent tendencies to make the diameter of a toner particle smaller in order to meet the demand for high-quality image formation require smaller carrier particles. However, the downsizing of the carrier particles could lead to reduction in the retention of each carrier particle. Effective measures are required to prevent the scattering of the carrier.
Among the disclosed various techniques relating to the carrier core particle, Japanese Unexamined Patent Application Publication No. 2008-241742 (PL1) discloses a technique with the aim of preventing the carrier from scattering.